Intellectual ability is a complex phenomenon, influenced by
factors both environmental and biological. Used in its
broadest sense, intelligence is what people use to learn,
remember, solve problems and in general deal effectively with
the world around them. One survey in the late 1980s asked 25
experts to define intelligence and the researchers received
many different definitions some of which are:

Francis Galton was among the first to measure individual
differences in intellectual abilities. Based on quantitative
studies of prominent individuals and their family trees, he
concluded that intellectual ability is inherited in much the
same way as physical traits, and he later published his
findings in Hereditary Genius (1869). Eventually, Galton
modified his original theories to recognize the effects of
education and other environmental factors on mental ability,
although he continued to regard heredity as the preeminent
influence.

Galton's work was followed in 1905 by that of French
psychologist Alfred Binet, who introduced the concept of
mental age, which would match chronological age in children of
average ability. It would exceed chronological age in bright
children and would be below in those of lesser ability.
Binet's test was introduced to the United States in a modified
form in 1916, and with it the concept of the intelligence
quotient (mental age divided by chronological age and
multiplied by 100). For example, if a 6-year-old girl scored a
mental age of 9, she would be assigned an IQ of 150
((9/6)x100).

The dominant method of studying intelligence in this century
has been the psychometric approach. Researchers using this
approach believe that intelligence can be measured through the
administration of various forms of IQ tests. These tests give
a score that reflects how far the person's performance
deviates from the average performance of others who are the
same age. Most modern tests arbitrarily define the average
score as 100.

Whatever the nature of the IQ test, many researchers believe
that a general statistical factor can be extracted from the
results of multiple IQ tests mainly due to the fact that
people who perform well on one type of intelligence test tend
to do well on others also. Charles Spearman named the general
mental ability that carried over from one test to another "g"
for general intelligence, and decided that it consisted mainly
of the ability to infer relationships based on one's
experiences.

In the 1960s American psychologists Raymond Cattell and John
Horn applied new methods of factor analysis and concluded
there are two kinds of general intelligence: fluid
intelligence (gf) and crystallized intelligence (gc). Fluid
intelligence represents the biological basis of intelligence.
Measures of fluid intelligence, such as speed of reasoning and
memory, increase into adulthood and then decline due to the
aging process. Crystallized intelligence, on the other hand,
is the knowledge and skills obtained through learning and
experience. As long as opportunities for learning are
available, crystallized intelligence can increase indefinitely
during a person's life. For example, vocabulary knowledge is
known to increase in college professors throughout their life
span.

Another approach is Howard Gardner's theory of multiple
intelligences, which replaces the general intelligence factor
(g ) with seven different types of intelligence: linguistic;
logical-mathematical; spatial; interpersonal (ability to deal
with other people); intrapersonal (insight into oneself);
musical; and bodily-kinesthetic (athletic ability). According
to Gardner, each of these areas of competence includes a
separate set of problem-solving skills that can be mobilized
by various symbolic systems. Every person has all the
different types of intelligence, although some may be
developed far more fully than others. (The most dramatic
example of this is found in savants, mentally retarded people
with exceptional abilities in a few highly specialized areas,
usually involving calculations.)

When you are born, your brain weighs
about a pound. But by age 6, it weighs three pounds. What
happens? Learning to stand, talk, and walk creates a web of
connections in your head—two pounds worth!

Your brain is full of nerve cells,
but it has no pain receptors. Doctors can operate on your
brain while you're awake and you won't feel a thing.

In 1984 the political scientist James
Flynn reported that Americans had gained about 13.8 IQ
points in 46 years. If people taking an IQ test today were
scored in the same way as people 50 years ago then 90% of
them would be classified in the genius level.

A message for action travels from
your brain to your muscles as fast as 250 miles per hour.